1. Field of the Invention
The present invention relates to a technical art of an optical disk, in which the digital data is recorded. More particularly, the present invention relates to a technical art of an optical disk, in which the digital data is recorded in high density by using a sampled servo system, and an optical disk reproducing apparatus for decoding the digital data from the present optical disk to reproduce it.
2. Description of the Related Art
In recent years, as a system to record and reproduce the digital data in high density in an optical disk, a pit edge multilevel recording system is well known, which records multilevel digital data by modulating a position of a pit edge on an optical disk in multi steps. A system is suggested such that this pit edge multilevel recording system and RPR (Radial Direction Partial Response) reproduction, which is a reproduction signal processing technique employing a partial response method, are combined (for example, it is disclosed in Japanese Patent Application Laid-Open No. 10-74322).
An example of a pit arrangement of an optical disk adopting such a system is shown in FIG. 10. In FIG. 10, on the tracks formed on the optical disk (in FIG. 10, they are denoted by T1 to T8), a synchronization pit Psync, a tracking pit Ptrk, a guard pit Pg and a data pit Pdat are arranged in a line. An area in which the synchronization pit Psync, the tracking pit Ptrk and the guard pit Pg are arranged constitutes a servo area, and an area in which the data pit Pdat is arranged constitutes a data area. In the case of adopting a RPR reproduction system, as shown in FIG. 10, a beam spot BS of a reproduction laser beam to be irradiated on the optical disk traces a centerline of two tracks.
According to the above described constitution, the synchronization pit Psync serves as a synchronization reference in reproduction. A pit length of the synchronization pit Psync is larger than the pit length of the other pits and the synchronization pits Psync are aligned in a disk radial direction so that the optical disk reproducing apparatus can easily detect the synchronization pit Psync. The synchronization pit Psync is also used as a clock phase reference in reproduction.
The tracking pit Ptrk serves as a tracking reference in reproduction. As shown in FIG. 10, the tracking pit Ptrk is provided in an area following the synchronization pit Psync, and the pit position has been changed by four-track period. For tracking servo control by the optical disk reproducing apparatus, reproduction signals of two tracking pits Ptrk located on the adjoining two tracks are used as a reference.
The guard pit Pg prevents the reproduction signals of the servo area and the data area from interfering each other and it is provided on a border between the servo area and the data area.
The data pit Pdat records the digital data by the pit edge multilevel recording system and constitutes a pit row with a given period on the track. Respective data pits Pdat are capable of recording the digital data with three values by changing a position of the pit edge in three steps. In FIG. 10, three positions of the pit edge are shown to be overlapped, but each actual pit edge takes one of three positions.
Next, waveforms of a reproduction signal when the servo area, its preceding area, and its succeeding area are reproduced from the optical disk having a pit position shown in FIG. 10 will be explained. FIG. 11 shows waveforms of a reproduction signal when the reproduction operation has been normally performed. In FIG. 11, waveforms of a reproduction signal are shown when the beam spot BS traces a centerline of a track T4 and a track T5 in FIG. 10. In FIG. 11, a horizontal axis denotes time and a scale in the horizontal axis represents sampling timing of the reproduction signal by a reproduction clock. A vertical direction denotes a reproduction signal level and the horizontal axis represents a center level of the reproduction signal. Additionally, when the beam spot BS reproduces a pit, the reflection light intensity is assumed to be decreased and its reproduction signal level is assumed to be lowered.
As shown in FIG. 11, if the beam spot BS reaches to the servo area to reproduce the synchronization pit Psync, a large negative peak p1 is obtained. Then, the beam spot BS proceeds to reproduce the tracking pit Ptrk of the track T5, a small negative peak p2 is obtained. Then, the beam spot BS proceeds to reproduce the tracking pit Ptrk of the track T4, a small negative peak p3 is obtained. As shown in FIG. 11, the negative peak p2 and the negative peak p3 are in an equal level.
In the tracking servo control in the optical disk reproducing apparatus, the reproduction signal level in accordance with two tracking pits Ptrk, which are located on the adjoining two tracks, is detected and the difference between two levels is detected as a tracking error. Accordingly, a tracking error TE in FIG. 11 is represented as TE=p2−p3=0. Therefore, when the optical disk reproducing apparatus assures normal reproduction operation, the beam spot BS can trace a centerline of two tracks accurately since tracking servo control performs so that the tracking error TE becomes 0.
However, the optical disk reproducing apparatus does not always assure the normal reproduction operation. At first, the inter-symbol interference occurring in the reproduction operation becomes a problem. This inter-symbol interference is signal waveform distortion due to interference among adjoining pits on the track. The inter-symbol interference occurs when an optical property of the optical disk changes in reproduction, a disk surface is blurred or distorted, and a servo error or an electrical property of the optical disk reproducing apparatus changes. Secondly, the sag occurring in the reproduction operation becomes a problem. A HPF (High Pass Filter) is generally provided in an amplifier circuit of a reproduction signal in order to attenuate a low frequency component of the reproduction signal. However, if the reproduction signal from the optical disk has a low frequency component, the HPF distorts the signal waveform, namely, the sag occurs.
FIG. 12 shows waveforms of a reproduction signal when the same area as that shown in FIG. 11 is reproduced. These are waveforms of a reproduction signal in the case that the inter-symbol interference described above occurs in the reproduction operation. The tracking pit Ptrk of the track T5 in FIG. 10 is arranged in the vicinity of the synchronization pit Psync, so that the interference of the synchronization pit Psync becomes stronger. Hereby, the reflection light intensity is decreased on a position of the tracking pit Ptrk, so that the level of the negative peak p2 is lowered. On the other hand, the tracking pit Ptrk of the track T4 in FIG. 10 is located far from the synchronization pit Psync, so that the interference of the synchronization pit Psync becomes weak. Hereby, the reflection light intensity on a position of the tracking pit Ptrk is not influenced, so that the level of the negative peak p3 is not changed. Accordingly, a tracking error TE in FIG. 12 is represented as TE=p2−p3<0.
Next, FIG. 13 shows waveforms of a reproduction signal when the same area as that shown in FIG. 11 is reproduced. These are waveforms of a reproduction signal in the case that the sag is generated in the reproduction operation. Since the pits in the servo area of the optical disk in FIG. 10 are sparser compared to the pits in the data area, an average level of the reproduction signal becomes higher. Therefore, the reproduction signal has a low frequency component equal to the frequency of the servo area, so that the sag is subjected to occur.
In the case that the sag occurs in the reproduction signal, as described above, the waveform is changed with time so that the average level approaches to zero. Accordingly, while the levels of the preceding negative peak p1 and the negative peak p2 are not changed, a level of the negative peak p3 that appears after certain time has passed is lowered. Namely, a tracking error TE in FIG. 13 is represented as TE=p2−p3>0.
As described above, when the waveform of a reproduction signal is distorted by the inter-symbol interference or the sag in the reproduction of the optical disk, the tracking error TE does not become 0 even if the beam spot BS traces a centerline of two tracks. In other words, as described above, if the inter-symbol interference is generated, TE becomes lower than 0, and if the sag is generated, TE becomes higher than 0. If the tracking servo is performed on the basis of the inaccurate tracking error TE in the optical disk reproducing apparatus, the beam spot BS is displaced from the centerline to be traced and so called tracking offset is generated. In this way, according to a conventional system, there is a problem that the tracking servo control becomes inaccurate as a result of the inter-symbol interference and the sag occurring in the reproduction.